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An ultralight scalar field is a candidate for the dark matter. The ultralight scalar dark matter with mass around $10^{-23},{rm eV}$ induces oscillations of the pulse arrival time in the sensitive frequency range of the pulsar timing arrays. We search for the ultralight scalar dark matter using the North American Nanohertz Observatory for Gravitational Waves 11-year Data Set. We give the 95% confidence upper limit for the signal induced by the ultralight scalar dark matter. In comparison with the published Bayesian upper limits on the amplitude of the ultralight scalar dark matter obtained by Bayesian analysis using the Parkes Pulsar Timing Array 12-year data set (Porayko et al. 2018), we find three times stronger upper limit in the frequency range from $10^{-8.34}$ to $10^{-8.19},{ rm Hz}$ which corresponds to the mass range from $9.45times10^{-24}$ to $1.34times10^{-23},{rm eV}$. In terms of the energy density of the dark matter, we find that the energy density near the Earth is less than $7,{rm GeV/cm^3}$ in the range from $10^{-8.55}$ to $10^{-8.01},{ rm Hz}$ (from $5.83times10^{-24}$ to $2.02times10^{-23},{rm eV}$). The strongest upper limit on the the energy density is given by $2,{rm GeV/cm^3}$ at a frequency $10^{-8.28},{ rm Hz}$ (corresponding to a mass $1.09times10^{-23},{rm eV}$). We find that the signal of the ultralight scalar dark matter can be explained by the solar system ephemeris effect. Also, we reveal that the model of the solar system ephemeris effect prefers parameters which are contrary to the expectation that noise will be reduced on all pulsars.
It is widely accepted that dark matter contributes about a quarter of the critical mass-energy density in our Universe. The nature of dark matter is currently unknown, with the mass of possible constituents spanning nearly one hundred orders of magni
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The regularity of pulsar emissions becomes apparent once we reference the pulses times of arrivals to the inertial rest frame of the solar system. It follows that errors in the determination of Earths position with respect to the solar-system barycen